All provinces have a population type that determines what types of units and commanders can be recruited there. If a fortress is built in the province, the owning nation's national troops are added to the population type's troops, except for capitol only troops. Some independent troops and commanders require a temple or magic laboratory for recruitment.
The population type in a nation's own capitol is ignored (the nation can recruit only its own national troops there), but if the capitol is conquered by another nation, the conquering nation gains access to the population type of the conquered capitol in addition to the conqueror's national troops (since a fortress is automatically present). They do not usually get access to any of the conquered nation's national troops.
In Dominions 3, the distribution of population types through the world is different depending on the Era the game takes place in. Complete lists are not yet available, but it is believed from preview information that primitive barbarian tribes and amazons are more common in earlier eras, while feudal provinces with knights, longbows and even crossbows appear in later eras. Population type is also influenced by the terrain of the province.
Sea provinces have a separate list of population types, which consists of aquatic and amphibious units. Some amphibian population types are also found in coastal provinces, which can be helpful to land nations seeking to invade the oceans.